1. Field of the Invention
The present invention relates to pumping voltage generators, and more specifically, to a pumping voltage generator configured to perform a proper pumping operation using multiple detection levels, thereby reducing power consumption.
2. Description of the Related Art
Generally, a voltage VPP higher than power voltage of a memory cell array in a DRAM is used to prevent a threshold voltage of a NMOS transistor in a wordline from falling. Particularly, since a low voltage DRAM requires a high voltage due to its speed and characteristics, a circuit for generating a high voltage VPP is required.
FIG. 1 is a block diagram illustrating a conventional pumping voltage generator. The conventional pumping voltage generator comprises a detector 1, an oscillator 2, a control driver 3, and a pumping unit 4. The conventional pumping voltage generator has a feed-back structure wherein a pumping voltage VPP outputted from the pumping unit 4 is re-inputted into the detector 1.
The detector 1 detects a potential of the pumping voltage VPP from the pumping unit 4, and outputs a pumping oscillation enable signal VPP_EN in response to the detected potential.
FIG. 2 is a circuit diagram illustrating the detector 1 of FIG. 1.
Referring to FIG. 2, the detector 1 comprising resistances R1, R2, R3 and R4, and a differential amplifier 5 compares the pumping voltage VPP with a target level. The resistances R1, R2, R3 and R4 connected in series between the pumping voltage VPP and a ground voltage VSS divide a voltage.
The differential amplifier 5 comprises PMOS transistors P1 and P2, NMOS transistors N1, N2 and N3, and an inverter I1. The PMOS transistors P1 and P2 are formed as a current mirror type. The NMOS transistors N1 and N2 amplify a value obtained by comparing a reference voltage VRC with the divided voltage by the resistances R1, R2, R3 and R4, and then output an amplified value. The amplified value is inverted by the inverter I1.
FIG. 3 is a circuit diagram illustrating the oscillator 2 of FIG. 1.
The oscillator 2 periodically outputs a pulse signal for controlling a pumping operation in response to a pumping oscillation enable signal VPP_EN outputted from the detector 1.
As shown in FIG. 3, the oscillator 2 comprises a NAND gate ND1, and inverters I2˜I5 connected in series. The oscillator 2 has a feed-back structure wherein an output signal from the inverter 5 is re-inputted into the NAND gate ND1.
The control driver 3 connected to an output terminal to the oscillator 2 outputs a driving control signal depending on the oscillation cycle outputted from the oscillator 2. The pumping unit 4 connected to an output terminal of the control driver 3 pumps charges in response to the control signal outputted from the control driver 3, and then outputs a pumping voltage VPP.
In this way, the conventional pumping voltage generator outputs a pumping voltage by pumping charges according to a predetermined oscillation cycle regardless of the amount of charge consumption.
FIG. 4 is a graph showing the operation of the conventional pumping voltage generator.
Referring to FIG. 4, the pumping voltage VPP shows a sine curve based on the target level. At a time of t1, the detector 1 detects the target level. The period from t1 to t2 is response time after detection of the target level. At a time of t2, the detector 1 detects when the pumping voltage VPP is lower than the target level, and the pumping unit 4 starts pumping charges.
The pumping operation is started from the time of t2, and finished at a time of t3. During the period from t3 to t4, the pumping voltage VPP is dropped by the consumption of charges. As a result, the conventional pumping voltage generator detects the VPP target level to prepare a pumping operation.
As described above, the conventional pumping cycle is regular, and its width is determined to prepare against when charges of the pumping voltage VPP are most consumed. Since the pumping voltage VPP has the different amount of consumed charges depending on various operations such as active or refresh operation, the resultant pumping cycle should vary. However, an oscillation cycle is determined by the largest amount of charge consumption in the conventional pumping voltage generator. As a result, unnecessary power consumption is generated by an excessive pumping operation in most operations. This excessive pumping operation generates noise because the variation width of the pumping voltage VPP becomes large.